Lamps such as high-power metal halide lamps are favored for applications in which a steady, very bright point-like source of light is required. For example, many projection systems use a high-pressure mercury-vapor metal-halide arc lamp, comprising a discharge chamber, or ‘burner’, in a quartz glass envelope capable of withstanding high temperatures. The burner contains a filling consisting of one or more rare gases, and, in the case of a mercury-vapor discharge lamp, mainly of mercury. By applying a high voltage across two electrodes (generally tungsten) protruding into the burner, a light arc is generated between the tips of the electrodes, which can then be maintained at a lower voltage. Optical properties such as a natural spectral composition and a high luminous intensity can best be achieved with high-intensity discharge (HID) lamps and, in particular, ultra-high-performance (UHP) lamps.
The temperature in the burner of such a lamp easily reaches several hundreds of degrees Celsius, and a high operating temperature is required for the halogen cycle to function correctly. At these high temperatures, tungsten evaporates from and is re-absorbed by the electrodes (tungsten transport). Convection currents in the burner result in the top of the burner being hotter than the bottom (here and in the following, the term ‘top’ is to be understood to be the uppermost region in the burner, while the term ‘base’ is used to refer to a lower or bottom region in the burner). However, the temperature in the lamp should not be allowed to increase too much, since the quartz of the burner wall will crystallize when subject to very high heat for prolonged length of time. For this reason, a lamp in a device such as a projector is generally cooled during operation, for example by directing a cooling airflow at the lamp, and the cooling airflow is directed primarily at the top or upper region of the lamp, so that this hotter region is cooled to a greater extent than the lower regions. This type of cooling is referred to as an ‘asymmetrical cooling’. Cooling is generally controlled so that the difference or delta between top and bottom burner temperature for a high-power halogen lamp lies within a certain span, for example 50° K to 100° K for a UHP lamp type. However, it may be advantageous to be able to operate a projector in different vertical orientations and not just a single, fixed position. A portable projector that is used in a ceiling position (e.g. for showing movies) could also be used in a desktop position (e.g. for a presentation), or vice versa. A fixed asymmetrical cooling of the type described above, designed for use only in a certain position (either ceiling or desktop) is unsuitable in such cases, since the cooling airflow is directed primarily towards the base of the burner when the projector is in the ‘wrong’ position, so that the base is cooled too much and the critical upper region is not cooled enough. As a result of the low temperature in the base, the mercury vapor condenses here. This has a detrimental impact on the functionality of the halogen cycle. When an excessive amount of mercury condenses as a result of over-cooling the base, the halogen will dissolve in that liquid mercury. As a result, not enough of the volatile halogen (e.g. bromine) is available in the rest of the burner to prevent the evaporated tungsten from being deposited as black matter on the inside of the burner wall, a process referred to as ‘blackening’. While these black deposits can settle anywhere on the inside wall of the burner, most are brought by convection currents to the top of the burner. However, this is also the hottest region in the lamp, so that the resulting darkened area will absorb heat and become even hotter, ultimately leading to crystallization of the quartz in that area, visible as a white discoloration of the quartz glass. Crystallization or ‘whitening’ is a relatively slow but irreversible process that leads to unsatisfactory performance and possibly even lamp failure or explosion.
To avoid such serious problems, some prior art solutions use a complex mechanical system allowing the cooling air stream to be re-directed, according to the position of the projector (desktop or ceiling), towards the top of the burner. Other systems have two fans, one for each ‘end’ of the burner, and a controller to select the fan speeds according to the position of the projector. This must be made known somehow, for example by relying on the user to select a certain input configuration, or by incorporating a motion sensor in the projector which can distinguish between an ‘upward’ (desktop) and ‘downward’ (ceiling) vertical orientation. However, such solutions add to the overall complexity and therefore the expense of a projector, and the solutions which rely on user input are subject to error.
Therefore, it is an object of the invention to provide an alternative, more efficient and more economical way of cooling a lamp.